This invention relates to a carburetor valve rotational setting retainer assembly, and more particularly to a rotational setting retainer assembly for low and high-speed needle valves of a carburetor for a combustion engine.
Government agencies of an increasing number of countries are imposing exhaust emission control regulations to protect the environment. These regulations are being applied to all combustion engines including portable or two cycle engines used in common equipment such as chain saws, lawn mowers and hedge trimmers. One means of limiting excessive exhaust emissions in a small engine is to restrict the maximum amount of fuel delivered to the combustion chamber. This maximum fuel amount is preset on each individual engine by the engine manufacturer with the understanding that the end user requires some adjustment capability to meet changing work conditions and environmental factors such as altitude. The higher the altitude, the lower the air density, and the lower the fuel amount necessary to operate the engine. The user of the engine must therefore be able to adjust the fuel to air mixture ratios and may do so via low and high-speed needle valves protruding from the carburetor.
Not only is it desirable to limit the richness of the fuel to air mixture because of exhaust emission regulatory concerns, but the engine manufacturer of a portable combustion engine product also wants to restrict minimum amounts of fuel, or the leanness of the fuel to air mixture. Often a user will desire more power from a small engine and will attempt to operate the engine in an ultra-lean state. This will deprive an engine of proper cooling and will lead to warranty concerns. Therefore, limiter caps are designed not only to restrict the carburetor to a maximum amount of fuel, but also to restrict the carburetor to a minimum amount of fuel.
Not only is it desirable to limit the maximum and minimum amounts of fuel, but it is also desirable to hold steady the fuel flow in a running engine. Any rotation of the needle of the needle valve, possibly caused by the vibration of a running engine would alter the fuel flow. Therefore, it is desirable to restrain the rotation of the needle of the needle valves thereby preventing any unintended changes to the fuel flow setting. Traditionally, compressed springs are disposed concentrically about the needle and axially between the carburetor body and the head of the needle valve. The spring induced axial force produces increased frictional forces amongst the threads between the carburetor body and the needle, thus resisting needle rotation and alteration of the fuel flow setting.
Unfortunately, engine vibration is not the only source of unintentionally altered fuel flow. Lateral wobble and axial shifting of the needle tip, disposed within an orifice of the carburetor fuel feed channel, can cause fuel flow changes resulting in a rough running engine. Furthermore, the factory prescribed setting of the low and high-speed needle valves can be rotatably and axially altered when the limit caps are applied to the heads of each needle. For further background information on needle tip wobble, see U.S. patent application Ser. No. 09/584,970 filed on Jun. 1, 2000 which is incorporated by reference herein.
A retaining assembly maintains the factory pre-set fuel flow settings during the later attachment of a limiter cap to a fuel needle valve of a carburetor. A retainer disposed outward from the carburetor body laterally biases the fuel needle valve which increases frictional forces between the adjustment threads of the needle and carburetor body. The retainer also provides rotation resistant friction between the valve and the retainer itself. Preferably, the carburetor has a pair of spaced-apart and generally parallel low and high-speed needle valves. However, the carburetor may have a single fuel needle valve and a parallel rod cooperating with the retainer to inhibit rotation of the single valve.
Each valve has a needle which adjustably threads to the carburetor body. A shank of the needle protrudes from the carburetor body and engages concentrically a radially enlarged head at the distal end. Restraining rotation of the needle by exerting an axial force is a spring compressed concentrically between the head of the needle and the carburetor body. Restraining rotation of both needles by exerting a lateral force is a retainer aligned generally axially between the carburetor body and the heads of the needles, and preferably disposed radially outward from the springs of the low and high-speed needle valves.
Preferably, the needles have a needle tip which resides within a fuel flow orifice of the carburetor body. Both axial and lateral movement of the tip relative to the orifice respectively changes fuel flow into the throttling bore or mixture chamber. The retainer produces bending stresses and strains within the needles of both valves which propagate longitudinally down the needle to the tip. The tip is thereby biased laterally toward a side of the orifice.
Objects, features and advantages of this invention include the elimination of needle tip wobble which adversely effects fuel flow, providing a simple and inexpensive means to restrain rotation of the low and high speed needle valves, and facilitating and preserving final fuel flow adjustment of the carburetor.